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11 thoughts on “You learn something new every day”

Something I found out when I upgraded from 158gr SJHP in a .357 Magnum, to the Cor-Bon 125gr stuff.

The techincal term for this is called “jump”.

Interestingly enough, the Brits and Canadians thought this was important enough information that they put a section on factory sight regulation (including an extensive discussion of “jump”) in the basic recruit guides, in the middle of the darkest days of World War II.

They dumped a lot of “useless’ crap out of the training syllabus, and simplified drill (temporarily, for the duration) in order to cut down on how long it took to train a new private.

But they felt that discussing “jump” and how the factory armorers will replace the front sight with one of a specific height to account for it was critical information necessary to have a soldier (not even just infantrymen) who was competant with his rifle and confident in it’s accuracy.

I forgot to specify — “jump” is the movement of the muzzle from the moment the round fires until the bullet leaves the barrel, and the resulting displacement from point of aim which needs to be compensated for so the gun will shoot point of aim – point of impact at the specified zero range.

If you fail to account for jump, even if you have a perfect ballistic table and can readily do teh trigonometry to figure out the sight alingment necessary to bring your point of aim into that ballistic path at the right point, you will ALWAYS hit high.

Reducing bullet velocities has the same (albeit much smaller, as the factory already compensated for some of the total jump) effect — you have not totally acocunted for jump.

And of course, increasing bullet velocities is the same as overestimating jump.

Of course, at reasonable defensive ranges, the realistic combat accuracy you could expect when it’s for real makes this an academic point most of the time. It’s easy enough to compensate for the difference if you know how your gun shoots with your ammo at teh ranges involved — and if you need such accuracy that misisng by the amount of jump is a failure, you’ll be taking enough time in sight picture you can realistically hold off the required amount.

Which is why fixed sights work for carry guns, even if you use different ammo than they are factory regulated for. It’s really the same as if your gun is zeroed at a different range than the factory thinks.

A friend told me about this and I experienced it first hand shooting 700gr loads out of his SW500. It’s especially important on the x-frames because of the wide ranges of bullet weights and velocities.

With the gun sighted for 385gr at 1650fps, the 700gr at 1150fps hits 16″ high at 40 yards.

Likely so. If your handloads are loaded so the internal ballistic velocities are comparable to the factory 130 grain loads (which are, as far as I know, “standard” pressure loads), the jump would be comparable. (Keep in mind that the difference in recoil from different bullet weights is going to affect jump as well — if your gun rolls up faster with the reload, that will compensate for SOME of the difference.)

Of course, that just means you are ballistically matched at the range your gun is zeroed for. Any other range can be an entirely different matter.